44 



EARLY AMPHIBIAN DEVELOPMENT 



minative effect to differences between the nuclei. The position of 

 any given nucleus in one or another presumptive region is without 



Fig. 14 



The development of regions by dependent differentiation during the stage of 

 plasticity, before the onset of irreversible determination. At the early gastrula 

 stage, a piece of presumptive epidermis (gill region) of Triton cristatus is 

 exchanged for a piece of presumptive brain region of T. tae?iiatus. a, The dark 

 taeniatus embryo with the light cristatus graft, h. The light cristatus embryo 

 with the dark taeniatus graft, c, The taeniatus embryo at a later stage, with the 

 cristatus graft in the region of the left side of the brain, d. Transverse section 

 through the taeniatus embryo, showing part of the wall of the forebrain (between 

 X-X) formed from the grafted light-coloured cristatus tissue, which has under- 

 gone dependent differentiation according to its surroundings. (From Spemann, 

 Arch. Entwmech. XLViii, 1921.) 



effect on the subsequent normal differentiation of that region. The 

 key to the origin of the differentiation of the various regions must 



